Download
1 / 19
190 likes | 310 Vues
This resource provides an overview of human heredity, focusing on chromosomes, genetic traits, and blood types. It explains the structure of chromosomes, including sex and autosome pairs, and how these relate to offspring sex determination. The complexity of blood types, including dominance, codominance, and recessive traits, is discussed, with examples of how parental blood types can result in various offspring types. Additionally, it outlines the role of pedigree charts in tracing genetic traits, and delves into genetic disorders such as cystic fibrosis and sickle cell disease.
E N D
Chromosomes • We discussed earlier in this unit, the usage of karyotype charts • Humans have 23 pairs of chromosomes • 1 sex chromosome pair • 22 autosome pairs • Recall that males have one X and one Y, while females have two X’s • These split during meiosis, and we can use a Punnett square to determine the possible outcomes for sex of offspring
From this, we can see that the mother can only supply an X chromosome • The sex of the child is solely due to the father • Sperm cells should be produced in equal numbers for X and Y
Human Genetics • Recall human blood type has multiple alleles • The traditional convention for expressing dominance and recessiveness no longer works • Alleles are often expressed as superscripts • Both A and B types are codominant, and O is recessive • A is IA, B is IB, and O is i • The following chart summarizes the genotypes and phenotypes
Because A and B are codominant, they both show up, but do not blend • The individual will be AB • A lack of either results in O
Example • A male heterozygous for blood type A plans to have children with a female who is heterozygous for B. What possible blood types could their offspring have?
IBi IB i IAIB IAi IA IAi i IBi ii
The possible blood types for the offspring would be AB, A, B or O • Additionally, the Rh+ factor is a dominant allele
Pedigree Charts • After identifying the nature of a trait, geneticists often look at family history • By understanding the phenotypes of certain members of a family, they can gather more info about others • This is organized in a pedigree chart
Human Traits A square represents a male. A circle represents a female. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and a female represents a marriage. A shaded circle or square indicates that a person expresses the trait. A circle or square that is not shaded indicates that a person does not express the trait.
These only work for traits that are thought to be controlled by genetics alone • Also works best on traits the are due to one gene
Genetic Disorders • Many conditions are due to recessive alleles • These will only manifest themselves if a dominant allele is present • An example is cystic fibrosis (CF)
Cystic Fibrosis • Caused by a recessive allele • Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways
The most common allele that causes cystic fibrosis is missing 3 DNA bases. • As a result, the amino acid phenylalanine is missing from the CFTR protein.
From Mutation to Disease • Normal CFTR is a chloride ion channel in cell membranes • Abnormal CFTR cannot be transported to the cell membrane • If it does, it will not transport Cl- as easily • Part 1 • Part 2
Other conditions arise from codominant alleles • In these cases, the heterozygotes have a different phenotype • You saw this with thalassemia in question 12 • Sickle cell disease is another example • Individuals that are heterozygous for this usually have normal blood cells, but are resistant to malaria
Finally, some conditions are caused by dominant alleles, although it is uncommon
